Non-destructive inspection of metallic materials, for example materials from which structural engineering components are composed, can be undertaken by introducing ultrasound into the material. Information about defects within the material can then be obtained by receiving and analysing the ultrasound signal after it has travelled within the material.
Ultrasound is commonly introduced into a material by means of piezoelectric transducers, which have a face that vibrates mechanically at ultrasound frequencies. The ultrasound is normally passed into the material to be tested by a coupling medium, for example water, which is introduced between the transducer face and the material under test.
For the ultrasonic testing of some materials, it is impractical, or in some cases disadvantageous, to introduce a coupling medium, for example during the inspection of gas pipelines by a moving inspection vehicle, commonly referred to as an inspection pig. Under these circumstances a means of dry-coupling the ultrasound into the material is required.
Ultrasound can be excited in electrically conductive materials by the application of a high frequency magnetic field in the presence of a second magnetic field which is permanent or varies very slowly. Devices for achieving this are called electromagnetic acoustic transducers (hereinafter referred to as EMATs).
For EMATs used in conjunction with a permanent magnetic field, the usual means of operation is that the high frequency magnetic field created by the EMAT produces electrical eddy currents within the material. These eddy currents flow in the presence of the permanent magnetic field and generate lorentz forces acting within the material. These forces create mechanical displacements within the material, which propagate as acoustic waves. In the ferromagnetic material there are two other mechanisms by which the high frequency magnetic field initiates acoustic waves, namely magnetostriction and magnetic body forces. These additional forces can also play a part in the initiation of acoustic waves.
Numerous designs for EMATs have been proposed each of which is characterised by a particular geometry for the two key components of the EMAT, namely the mechanism for generating a permanent magnetic field in the test specimen and the electrical winding used to carry the high frequency electrical current. The result of altering these components of the EMAT can be that the ultrasound introduced into the test specimen is altered in the direction it is radiated or in the propagation mode, for example compression mode or transverse (shear) mode.
Nearly all EMAT transducers suffer from ‘barkhausen’ noise when moved over the surface of a ferromagnetic system. Barkhausen noise is due to the discontinuous motion of ferromagnetic domain boundaries during changes in the bulk magnetisation of ferromagnetic material below the saturating field for the material. It is a consequence of the magnetising components within the EMAT transducer. Barkhausen noise can be a severe problem for EMAT pipe inspection using pigs, because the inspection is carried out at speed.
There are a variety of compromises made in the design of any EMAT, but the arrangement of the magnetic field components and the electrical windings can only be manipulated within limits set by the fundamental physical method of their operation. Each type of EMAT has its fundamental arrangement of magnets and windings. Once this has been selected, usually so as to create a desired wave mode, other aspects of the design, such as size, thickness of wear plate, mass, rigidity, heat tolerance and power handling, can be decided. These are practical choices that are constrained by the operating environment, and often limit the acoustic performance of the transducer as measured by, for example, the acoustic output amplitude or the prominence of the barkhausen noise. The design problem is therefore strongly affected by the fundamental method of the EMAT operation, which in some environments may preclude any practical solution.